This invention relates to an automatic signal level control circuit for use in telephone sets.
The length of the subscriber's line interconnecting a telephone set and a telephone office is determined by the location of the subscriber's station and not constant. For this reason, the signal level of the speech varies for different subscribers. Accordingly, in a telephone system, it is desirable to assure adequate and uniform signal level, that is signal level at the time of speech irrespective of the length of the subscriber's line between the telephone set and the telephone office.
According to one prior art solution of this problem, the length of the subscribers line was detected by a varistor which responds to the variation in the voltage across input terminals of a telephone circuit for inserting an AC resistance corresponding to the variation in the voltage-resistance characteristic of the varistor in parallel with the telephone circuit, that is a loop for passing current to the telephone set. Such method has been used in type 1500D telephone sets manufactured by Western Electric Co., in U.S.A., and described by J. H. Ham et al. in an article "A Touch-Tone Caller for Station Sets, I.E.E.E. Transactions on Communication and Electronics Vol. No. 65, 1963 pp 17-24".
FIG. 1 of the accompanying drawing shows elements related to the automatic signal level adjusting circuit utilized in the type 1500D telephone set which comprises input terminals 11a and 11b of the telephone set, a conventional telephone set circuit 12 including a hybrid coil 13, a transmitter 14, a balancing network 15, a varistor 18 for side tone balance, and a receiver 16, a varistor 17 connected across terminals 11a and 11b for adjusting transmitting and receiving signal levels, an impedance 19 connected in series with varistor 17 for adding AC loss.
In the circuit, the adjustment of the signal level of the transmitting and receiving signals is effected in the following manner. Where the subscriber's station is closely located to the telephone office and hence where the length of the subscriber's line is short, the voltage across the input terminals 11a and 11b is high since the line loss is small. Accordingly, the AC resistance of the varistor decreases substantially, so that the additional AC loss impedance 19 is connected across terminals 11a and 11b thus suppressing the level of the speech signal. On the other hand where the subscriber's station is remotely located from the telephone office, the length of the subscriber's line increases thus increasing the line loss. Accordingly, the voltage across input terminals decreases and the AC resistance of the varistor increases greatly, so that the series impedance of the varistor 17 and the impedance 19 become high, thus decreasing the parallel loss.
However, the automatic signal level adjusting circuit described above has various defects as follows.
Firstly, the DC resistance of the telephone set is generally lower than the relay resistance in the telephone office. Accordingly, the voltage across the input terminals 11a and 11b does not vary in proportion to the length of the subscriber's line, so that the amount of loss inserted in parallel across the terminals of the telephone set is liable to be affected by the depression in the characteristic of the varistor 17.
Secondly, as the side tone balance has been effected by taking a case wherein the subscriber's line is long is taken as a reference, the side tone balance is upset in a telephone set located closely to the telephone office, thereby increasing the side tone. This greatly degrades the transmission performance. In order to solve this problem it is necessary to suitably correlate the characteristics of the varistors 17 and 18. However, it is difficult to prepare varistors having the correlative characteristic, thereby increasing the cost of manufacturing.
Thirdly, since the voltage-resistance characteristic of the varistor is non-linear, the speech would be deformed.
Fourthly, where an AC signal having the same frequency band as the voice signal is used as the dial signal of the telephone set, the distortion characteristic of the dial signal would be degraded with the result that the push button dial signal receiver installed in the telephone office would misoperate, thus causing miss dialling.
According to another prior art technique the variation in the current supplied to the telephone set is detected to automatically adjust the signal level. This technique is described, for example, in U.S. Pat. No. 3,582,564 to Knauer dated June 1, 1971. According to the circuit arrangement disclosed in this patent, a current detection resistor is inserted in series with the loop for supplying current to the telephone set, the base and emitter electrodes of a transistor are connected across the current detection circuit and the collector and base electrodes are connected in parallel with the receiver or the transmitter via a capacitor.
According to this arrangement, instead of detecting the voltage across the input terminals of a telephone set the current supplied to the telephone set is detected and the amount of loss afforded by the nonlinear characteristic of a transistor is adjusted in accordance with the detected value of the current. Consequently, the variation in the amount of added loss caused by the difference in the characteristics of the elements used is smaller than the case of using varistors. However, in this arrangement too, since the nonlinear characteristic of the transistor is used for adjusting the amount of the added loss, where a push button dial is used, the dial signal would be distorted by the non-linear characteristic of the transistor thereby causing a miss dialling.
Furthermore, according to this arrangement, since the length of the subscriber's line is discriminated by the magnitude of the supply current, although it is possible to increase the sensitivity of adjusting the signal level than in the case of utilizing the varistor, the naturality of the speech would be impared because signal level adjusting circuit operates in response to the magnitude of an AC signal such as voice signal.